Johannes-Andreas Hanni was born on March 24, 1957, in Valga, which was then part of Soviet Estonia. He grew up in a family that followed the Baptist faith. His father, Jaan Hanni, served as a minister, and Johannes later claimed that he faced harsh treatment from his parents. This relationship caused him to develop feelings of hatred toward his father.

During his teenage years, Johannes had many troubles with the law. He was often in trouble for committing crimes and spent a significant amount of time in prisons and reformatories. This troubled background shaped much of his early life and influenced his future.

In 1981, Johannes married Pille Toomla, who worked as a trolley driver. The couple tied the knot on December 11. At that time, Johannes worked as a waiter at the Palace Hotel restaurant in Tallinn, Estonia. 

Johannes-Andreas Hanni committed his first murder on 5 March 1982. He killed Eimar Vibo, a sailor from Saaremaa, on Valdeku Street in Nõmme. After stabbing Vibo to death, Hanni cut off part of his thigh and took it home. He told his wife he wanted to eat it, saying he had wanted to try human flesh for a long time. He later fried the meat on the stove.

On 23 May 1982, Hanni stabbed Ivan Sivitsky, a 75-year-old pensioner from Belarus. This incident took place in his garden cottage sauna in Jõhvi. Hanni cut off Sivitsky's genitals, intending to use them as a sexual object. In late July 1982, Hanni raped and murdered Yevgenia Koltsova, a middle-aged Russian vagrant. She was picking raspberries in a forest area between Laagri and Saue. Later, in August 1982, Hanni and his wife planned to kill a random taxi driver in Tallinn. They attacked Alari Kivi on 2 September 1982 with a knife. Kivi fought back and received serious injuries, but they were not life-threatening.

Hanni was arrested on 2 October 1982. Before his court hearings, he underwent a psychiatric evaluation. Notable psychiatrist Anti Liiv reported that Hanni had unusual sexual preferences.

Johannes-Andreas Hanni hanged himself in prison on 6 November 1982, before he could be sentenced. His wife, Pille Hanni, was found guilty of being an accessory to the murders. She spent almost twelve years in Harku Prison before her release. After her release, she changed her name and moved to Finland.